Paper to be printed by a web-fed printing press is usually supplied in a roll which is mounted on a reel stand at the input end of the press and the paper is unwound continuously from the rollas the press requires it. The web of paper is preferably maintained under a constant predetermined amount of tension between the roll and the printing press, because if the tension is too high the web breaks and if it is too low, the web wanders laterally in the press. The tension in the web must be prevented from exceeding its breaking tension when the press is being stopped also.
A variety of apparatus is used in the prior art to maintain constant tension in the web between the reel stand and the press. For example, the tension is sensed by strain gauges at the bearings of an idler roll over which the web passes to change direction, and the torque of a brake or motor at the reel stand is controlled in accordance with the strain gauge signal.